The present invention relates to a device for protecting against an electrical overcurrent, for at least one electronic switching branch of an electrical conversion system, the or each branch comprising two switching half-branches serially connected at an intermediate terminal, at least one half-branch including at least one switching member, the or each switching member comprising a first controllable electronic switch and a diode connected in antiparallel to said first electronic switch.
The invention also relates to a system for converting a first electrical voltage into a second electrical voltage comprising such a protection device.
The invention also relates to a method for controlling such a protection device.
The field of the invention is that of electrical power conversion systems. The invention in particular relates to electricity production installations comprising at least one electricity conversion system, in particular systems for producing electricity from photovoltaic modules.
A protection device of the aforementioned type is known. Such a protection device is used within a voltage inverter connected between a photovoltaic module, providing a direct electrical current, and a three-phase alternating network. The voltage inverter includes three switching branches, each branch comprising two electronic switches and two diodes. Each diode is connected in antiparallel with an electronic switch. The protection device comprises several electrical contactors and/or several circuit breakers, each contactor and/or circuit breaker being connected serially between a phase output of the inverter and the electrical network.
In nominal value operation, the value of the input voltage of the inverter, on the side of the photovoltaic module, is greater than the value of the output voltage of the inverter, on the network side. In that case, the electrical current circulates from the photovoltaic module toward the network, via the inverter.
When a fault occurs, such as an electrical short-circuit on the side of the photovoltaic module, or when the electrical power at the input of the inverter decreases abruptly, the input voltage of the inverter decreases suddenly and drops below the peak voltage of the network. In that case, the electrical current circulates from the network toward the photovoltaic module, via the inverter. If the intensity of the current circulating in each diode of the inverter exceeds its nominal operating intensity, the diode may break. However, if the decrease in the input voltage of the inverter occurs over very short period of time, typically less than the characteristic time constant of the converter, the latter is too slow to be able to open the electrical circuit and thereby prevent the diode from breaking.
To avoid this phenomenon, a first known solution consists of connecting an impedance serially between each phase output of the inverter and the electrical network. The value of this impedance is chosen so as to limit the presumed short-circuit current corning from the network. However, this first solution causes an online voltage drop at the phase outputs of the inverter, which also requires overdimensioning the apparent power of the inverter. This solution consequently leads to a deterioration in the overall output of the conversion system and excess production costs.
A second known solution is to connect the diode serially between the output of the photovoltaic modules and the input of the inverter. This solution, although satisfying the response time requirements for protection of the diodes, is not, however, satisfactory in terms of the loss of output that it creates. In fact, the presence of the diode at the input of the inverter causes a drop in the input voltage, and consequently a deterioration of the overall output of the conversion system.
A third known solution consists of using fuses within the protection device. Each fuse is then connected serially between a phase output of the inverter and the electrical network. As for the second solution, the third solution is also not satisfactory in terms of output, the fuses having to reach a relatively high temperature to operate correctly. Furthermore, the use of fuses causes significant operating costs.